The Experts below are selected from a list of 125085 Experts worldwide ranked by ideXlab platform
Ronald Van Den Berg - One of the best experts on this subject based on the ideXlab platform.
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a resource Rational Theory of set size effects in human visual working memory
eLife, 2018Co-Authors: Ronald Van Den BergAbstract:You can read this sentence from beginning to end without losing track of its meaning thanks to your working memory. This system temporarily stores information relevant to whatever task you are currently performing. However, the more items you try to hold in working memory at once, the poorer the quality of each of the resulting memories. It has long been argued that this phenomenon – known as the set size effect – occurs because the brain devotes a fixed amount of neural resources to working memory. But this Theory struggles to account for certain experimental results. It also fails to explain why the brain would not simply recruit more resources whenever it has more items to remember. After all, your heart does something similar by beating faster whenever you increase your physical activity. Van den Berg and Ma break with the idea that working memory resources are fixed. They propose that resource allocation is flexible and driven by two conflicting goals: maximize memory performance, but use as few neural resources as necessary. Indeed, a computer simulation that follows this strategy mimics the set size effects seen in healthy volunteers. In the model, the items most relevant for a task are stored more accurately than less important ones, a phenomenon also observed in participants. Lastly, the simulation predicts that the total amount of resources devoted to working memory will vary with the number of items to be remembered. This too is consistent with the results of previous experiments. Working memory thus appears to be more flexible than previously thought. The amount of resources that the brain allocates to working memory is not fixed but could be the result of balancing resource cost against cognitive performance. If this is confirmed, it may be possible to improve working memory by offering rewards, or by increasing the perceived importance of a task.
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a resource Rational Theory of set size effects in visual working memory
bioRxiv, 2018Co-Authors: Ronald Van Den BergAbstract:Encoding precision in visual working memory decreases with the number of encoded items. Here, we propose a normative Theory for such set size effects: the brain minimizes a weighted sum of an error-based behavioral cost and a neural encoding cost. We construct a model from this Theory and find that it predicts set size effects. Notably, these effects are mediated by probing probability, which aligns with previous empirical findings. The model accounts well for effects of both set size and probing probability on encoding precision in nine delayed-estimation experiments. Moreover, we find support for the prediction that the total amount of invested resource can vary non-monotonically with set size. Finally, we show that it is sometimes optimal to encode only a subset or even none of the relevant items in a task. Our findings raise the possibility that cognitive "limitations" arise from Rational cost minimization rather than from constraints.
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a Rational Theory of set size effects in working memory and attention
bioRxiv, 2017Co-Authors: Ronald Van Den BergAbstract:The precision with which items are encoded in working memory and attention decreases with the number of encoded items. Current theories typically account for this "set size effect" by postulating a hard constraint on the allocated amount of encoding resource. While these theories have produced models that are descriptively successful, they offer no principled explanation for the very existence of set size effects: given their detrimental consequences for behavioral performance, why have these effects not been weeded out by evolutionary pressure, by allocating resources proportionally to the number of encoded items? Here, we propose a Theory that is based on an ecological notion of Rationality: set size effects are the result of a near-optimal trade-off between behavioral performance and the neural costs associated with stimulus encoding. We derive models for four visual working memory and attention tasks and show that they account well for data from eleven previously published experiments. Moreover, our results suggest that the total amount of resource that subjects allocate for stimulus encoding varies non-monotonically with set size, which is consistent with our Rational Theory of set size effects but not with previous descriptive theories. Altogether, our findings suggest that set size effects may have a Rational basis and highlight the importance of considering ecological costs in theories of human cognition.
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a Rational Theory of the limitations of working memory and attention
bioRxiv, 2017Co-Authors: Ronald Van Den BergAbstract:The precision with which items are encoded in working memory and attention decreases with the number of encoded items. Current theories typically account for this “set size effect” by postulating a hard constraint on the allocated amount of encoding resource, commonly formalized as samples, spikes, slots, or bits. While these theories have produced models that are quantitatively successful, they offer no principled explanation for the very existence of set size effects: given their detrimental consequences for behavioral performance, why have these effects not been weeded out by evolutionary pressure, for example by scaling the amount of allocated encoding resource with set size? Here, we propose a Theory that is based on an ecological notion of Rationality: set size effects establish an optimal trade-off between behavioral performance and the neural costs associated with stimulus encoding. We derive models from this Theory for four visual working memory and attention tasks and find that it accounts well for data in eleven different experiments. Our results suggest that set size effects have a Rational basis and that ecological costs should be considered in models of human behavior.
Dimitri Vayanos - One of the best experts on this subject based on the ideXlab platform.
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an institutional Theory of momentum and reversal
Review of Financial Studies, 2013Co-Authors: Dimitri Vayanos, Paul WoolleyAbstract:We propose a Rational Theory of momentum and reversal based on delegated portfolio management. A competitive investor can invest through an index fund or an active fund run by a manager with unknown ability. Following a negative cashflow shock to assets held by the active fund, the investor updates negatively about the manager’s ability and migrates to the index fund. While prices of assets held by the active fund drop in anticipation of the investor’s outflows, the drop is expected to continue, leading to momentum. Because outflows push prices below fundamental values, expected returns eventually rise, leading to reversal. Fund flows generate comovement and lead-lag effects, with predictability being stronger for assets with high idiosyncratic risk. We derive explicit solutions for asset prices, within a continuous-time normal-linear equilibrium.
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an institutional Theory of momentum and reversal
Social Science Research Network, 2008Co-Authors: Dimitri Vayanos, Paul WoolleyAbstract:We propose a Rational Theory of momentum and reversal based on delegated portfolio management. An investor can hold assets through an index or an active fund. Investing in the active fund involves a time-varying cost, interpreted as managerial perk or ability. The investor responds to an increase in the cost by flowing out of the active and into the index fund. While prices of assets held by the active fund drop in anticipation of these outflows, the drop is expected to continue, leading to momentum. Because outflows push prices below fundamental values, expected returns eventually rise, leading to reversal. Besides momentum and reversal, fund flows generate comovement, lead-lag effects and amplification, with all effects being larger for assets with high idiosyncratic risk. The active-fund manager's concern with commercial risk makes prices more volatile.
Paul Woolley - One of the best experts on this subject based on the ideXlab platform.
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an institutional Theory of momentum and reversal
Review of Financial Studies, 2013Co-Authors: Dimitri Vayanos, Paul WoolleyAbstract:We propose a Rational Theory of momentum and reversal based on delegated portfolio management. A competitive investor can invest through an index fund or an active fund run by a manager with unknown ability. Following a negative cashflow shock to assets held by the active fund, the investor updates negatively about the manager’s ability and migrates to the index fund. While prices of assets held by the active fund drop in anticipation of the investor’s outflows, the drop is expected to continue, leading to momentum. Because outflows push prices below fundamental values, expected returns eventually rise, leading to reversal. Fund flows generate comovement and lead-lag effects, with predictability being stronger for assets with high idiosyncratic risk. We derive explicit solutions for asset prices, within a continuous-time normal-linear equilibrium.
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an institutional Theory of momentum and reversal
Social Science Research Network, 2008Co-Authors: Dimitri Vayanos, Paul WoolleyAbstract:We propose a Rational Theory of momentum and reversal based on delegated portfolio management. An investor can hold assets through an index or an active fund. Investing in the active fund involves a time-varying cost, interpreted as managerial perk or ability. The investor responds to an increase in the cost by flowing out of the active and into the index fund. While prices of assets held by the active fund drop in anticipation of these outflows, the drop is expected to continue, leading to momentum. Because outflows push prices below fundamental values, expected returns eventually rise, leading to reversal. Besides momentum and reversal, fund flows generate comovement, lead-lag effects and amplification, with all effects being larger for assets with high idiosyncratic risk. The active-fund manager's concern with commercial risk makes prices more volatile.
Ron Giammarino - One of the best experts on this subject based on the ideXlab platform.
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corporate investment and asset price dynamics implications for seo event studies and long run performance
Journal of Finance, 2006Co-Authors: Murray Carlson, Adlai J Fisher, Ron GiammarinoAbstract:We present a Rational Theory of SEOs that explains a pre-issuance price run-up, a negative announcement effect, and long-run post-issuance underperformance. When SEOs finance investment in a real options framework, expected returns decrease endogenously because growth options are converted into assets in place. Regardless of their risk, the new assets are less risky than the options they replace. Although both size and book-to-market effects are present, standard matching procedures fail to fully capture the dynamics of risk and expected return. We calibrate the model and show that it closely matches the primary features of SEO return dynamics. THE ATYPICAL STOCK MARKET PERFORMANCE of public firms that issue seasoned equity raises an important challenge for financial Theory. Summarizing an extensive empirical literature,1 Ritter (2003) reports average stock market returns of 72% in the year prior to a seasoned equity offering (SEO), an announcement effect of -2%, and five-year post-issuance abnormal returns of about -30% relative to seemingly reasonable benchmarks. A comprehensive explanation
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corporate investment and asset price dynamics implications for seo event studies and long run performance
Social Science Research Network, 2004Co-Authors: Murray Carlson, Adlai J Fisher, Ron GiammarinoAbstract:We present a Rational Theory of return behavior around seasoned equity offerings, including a pre-issuance price runup, negative announcement effect, and long-run post-issuance underperformance. The main result uses real option principles to relate SEO's to an endogenous decrease in expected returns. Equity issues are associated with firm expansions. When firms invest, they convert growth options to assets in place. Even when the new assets are risky, they will be less risky than the options they replace. Although both size and book-to-market effects are present in our model, standard matching procedures fail to capture the dynamics of risk and expected return. We calibrate the model, and show that it gives a close match to the primary empirical moments.
Zarate Garnica Gabriela - One of the best experts on this subject based on the ideXlab platform.
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Analysis of shear transfer mechanisms in concrete members without shear reinforcement based on kinematic measurements
2018Co-Authors: Zarate Garnica GabrielaAbstract:Shear failure is one of the most critical failure modes of reinforced concrete members, especially for those without shear reinforcement. Despite the extensive research programs in the last decades, there is still no general agreement on a Rational Theory to assess their shear capacity. In the recent years, researchers have focused on developing mechanical models for shear design that are based on a predefined crack pattern and kinematics assuming that the shear force can be transferred through a critical shear crack by various shear transfer mechanisms. These mechanical models need to be validated by detailed kinematic measurements taken from experiments. This information could not be acquired before, however, in the recent years, new measurement technology has developed quickly, amongst others, Digital Image Correlation (DIC) provides new opportunities to obtain the crack pattern and kinematics through the displacement field of the whole surface of the target specimen. In this research, the possibility of using DIC measurements to apply a detailed analysis on the contribution of the shear transfer mechanisms (uncracked concrete, aggregate interlock, dowel action, and arch action) is explored in order to obtain a better understanding of the shear failure process. The analysis is based on ten representative tests on reinforced concrete beam specimens with a height of 1200 mm. The tests are selected from an experimental program designed to study the shear behaviour of reinforced concrete slab strips without shear reinforcement. A new algorithm is developed to automatically determine the contributions of the different shear transfer mechanisms along a crack from the displacement field obtained by DIC measurements. A comparison between the experimental results and the sum of the calculated contributions yield to a reasonable agreement, with an error of 40% when the shear failure is presented just after the formation of the flexural shear crack. With the help of the new algorithm, new insights on the three different shear failure modes observed in the experiments (flexural shear failure, shear compression, and dowel failure) are discussed. Flexural shear failure is attributed to the loss of aggregate interlock when a sudden increase of the shear displacement of the critical shear crack is observed. However, such decrease occurs earlier before the actual failure occurs. Therefore, the actual shear failure mechanism should be studied at an earlier stage. In a shear compression failure, the arch action turns out to be dominating. For members with large depth and exceptionally low reinforcement ratio (< 0.3%), a different failure mode may occur: dowel failure. Which is defined by the opening of the secondary branch of a major flexural crack along the tensile reinforcement. In the three failure modes, the importance of the crack opening in vertical and longitudinal direction is demonstrated, since the increase of them directly result in the drop of shear force that can be transferred through aggregate interlock. Finally, the results are a valuable input to further improve the Critical Shear Displacement Theory. It is suggested that the assumed simplified crack profile is modified to a crack with an angle between 60° to 70° to represent more accurately the results and to allow for larger shear displacements. It is demonstrated that an increase of the shear displacement is critical to triggering the shear flexural failure as proposed by this Theory and the actual value of the critical shear displacement is larger than the proposed value based on regression analysis.Structural Engineerin
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Analysis of shear transfer mechanisms in concrete members without shear reinforcement based on kinematic measurements
2018Co-Authors: Zarate Garnica GabrielaAbstract:Shear failure is one of the most critical failure modes of reinforced concrete members, especially for those without shear reinforcement. Despite the extensive research programs in the last decades, there is still no general agreement on a Rational Theory to assess their shear capacity. In the recent years, researchers have focused on developing mechanical models for shear design that are based on a predefined crack pattern and kinematics assuming that the shear force can be transferred through a critical shear crack by various shear transfer mechanisms. These mechanical models need to be validated by detailed kinematic measurements taken from experiments. This information could not be acquired before, however, in the recent years, new measurement technology has developed quickly, amongst others, Digital Image Correlation (DIC) provides new opportunities to obtain the crack pattern and kinematics through the displacement field of the whole surface of the target specimen. In this research, the possibility of using DIC measurements to apply a detailed analysis on the contribution of the shear transfer mechanisms (uncracked concrete, aggregate interlock, dowel action, and arch action) is explored in order to obtain a better understanding of the shear failure process. The analysis is based on ten representative tests on reinforced concrete beam specimens with a height of 1200 mm. The tests are selected from an experimental program designed to study the shear behaviour of reinforced concrete slab strips without shear reinforcement. A new algorithm is developed to automatically determine the contributions of the different shear transfer mechanisms along a crack from the displacement field obtained by DIC measurements. A comparison between the experimental results and the sum of the calculated contributions yield to a reasonable agreement, with an error of 40% when the shear failure is presented just after the formation of the flexural shear crack. With the help of the new algorithm, new insights on the three different shear failure modes observed in the experiments (flexural shear failure, shear compression, and dowel failure) are discussed. Flexural shear failure is attributed to the loss of aggregate interlock when a sudden increase of the shear displacement of the critical shear crack is observed. However, such decrease occurs earlier before the actual failure occurs. Therefore, the actual shear failure mechanism should be studied at an earlier stage. In a shear compression failure, the arch action turns out to be dominating. For members with large depth and exceptionally low reinforcement ratio (< 0.3%), a different failure mode may occur: dowel failure. Which is defined by the opening of the secondary branch of a major flexural crack along the tensile reinforcement. In the three failure modes, the importance of the crack opening in vertical and longitudinal direction is demonstrated, since the increase of them directly result in the drop of shear force that can be transferred through aggregate interlock. Finally, the results are a valuable input to further improve the Critical Shear Displacement Theory. It is suggested that the assumed simplified crack profile is modified to a crack with an angle between 60° to 70° to represent more accurately the results and to allow for larger shear displacements. It is demonstrated that an increase of the shear displacement is critical to triggering the shear flexural failure as proposed by this Theory and the actual value of the critical shear displacement is larger than the proposed value based on regression analysis.Civil Engineering | Structural Engineerin
